Ultrasonic atomiser



April 1968 H. M. BUTTERWORTH ETAL 3,375,977

ULTRAS ONIC ATOMISER Filed March 12, 1965 FIG.3

INVENTOR) HAROLD M. BUTTERWORTH ERNEST A. NEPPIRAS AGENT b United States Patent ()fifice 3,3 75,977 Patented Apr. 2., 1968 3,375,977 ULTRASONIC ATOMISER Harold Millman Butterworth, Crawley, and Ernest Arthur Neppiras, Reigate, England, assignors to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware Filed Mar. 12, 1965, Ser. No. 439,359 Claims priority, application GreatBritain, Mar. 25, 1964, 12,613/ 64 5 Claims. (Cl. 239-102) The invention relates to an ultrasonic atomiser, particularly to a fuel liquid atomiser for boilers.

In British patent specification 910,357 there is described an atomiser in which the liquid to be atomised is fed to the vibrating end face of a tuned velocity transformer, which is driven by an electromechanical transducer by injecting the liquid at a nodal point of the transformer into an inner bore which communicates with the end face.

A variant thereof is described in our co-pending US. application, Ser. No. 439,358 now Patent No. 3,285,517 in which the liquid to be atomised is fed internally of a sleeve surrounding the output limb of the transformer, so that the liquid flows along the limb towards the hemispherical end thereof.

The ultrasonic atomiser according to this invention comprises an electromechanical transducer which is in acoustic and mechanical contact with a velocity transformer formed by a body of revolution with a free vibrating end face, and means for feeding the liquid to be atomised to said face in the form of a thin skin applied to said face and is characterized in that the end face has the shape of a curved surface smoothly joining the velocity transformer, the radius of curvature of said face being initially at least equal to the radius of the velocity tranformer, where it terminates in the curved surface.

From experiments on which the invention is based it has appeared that, although the known devices operate satisfactorily for liquids in a given range of viscosities, they are inoperative to a greater or lesser extent for liquids of higher viscosity. In certain cases only considerably lower liquid flow rates can be attained by the same ultrasonic input power. Moreover, with high viscosities the size of the droplets may vary considerably. For fuel oil this non-uniformity may give rise to incomplete combustion.

It has been found that the shape of the vibrating free end or face of the velocity transformer, to which the liq uid is fed, has a marked effect on the atomising efliciency, as well as the manner in which the liquid is fed to said free face. For liquids having substantially the same viscosity as water the vibrating end face of the velocity transformer is substantially hemispherical, or semi-hemispherical, the liquid being fed to said face so that a thin skin or film is formed thereon.

For liquids having a higher viscosity than water it has been found that the vibrating face must have, in a longitudinal cross section, a more or less ogival form, so that the tangent to the ogive surface is, at any point, at a negative angle of inclination to the axis of revolution of the rotation-symmetrical surface of the ogive.

With a higher viscosity fuel it is found to be desirable to modify the shape of the vibrating free face in the proximity of said axis of revolution of the ogive, so that the tangent to the curved surface near or at the axis is substantially parallel to the axis, the tip portion of the free end of the face having the form of a projecting pip or point.

With still higher viscosity it appears to be desirable to modify the shape still further, so that the curved face displays a point of inflection, the tangent to the curve beyond this point having a positive angle of inclination to the axis of revolution of the curve, so that the surface displays a neck.

Further particulars of the invention will be evident from the following description given with reference to the accompanying drawing, in which:

FIG. 1 shows diagrammatically a velocity transformer with a vibrating end surface of a given shape.

FIG. 2 shows a modified form of the end surface to a larger scale and FIG. 3 shows a still further modified form of the end surface.

FIG. 1 shows diagrammatically a portion of a stepped cylindrical velocity transformer 1, having an input limb 2 and an output limb 3 of smaller diameter. The transformer is driven by an ultrasonic transducer (not shown) of any suitable known type, for example a nragnetostriction transducer.

The liquid to be atomised is fed to the vibrating end of the limb 3 by a plurality of feed pipes 4, 5, the open ends of which terminate closely adjacent the surface of the limb. The free end 10 of the limb 3, as is shown in the drawing, has more or less an ogival form for liquids of low viscosity, i.e. less than 10 centipoise, while the cylindrical portion of the limb 3 terminates smoothly with the curved surface of the ogive. The radius of curvature of the ogive has a value approximately equal to twice the radius of the cylindrical portion. Preferably the feed pipes are arranged so as to deliver the liquid to be atomised to the limb at the start of the curved end portion i.e. where the ogive meets the cylindrical portion of the: limb 3.

As stated above, it has been found that the curvature of the vibrating end surface 10 of limb 3 of the transformer, in order to obtain an effective atomisation, de pends upon the viscosity of the liquid and upon the quantity of acoustical energy of the said surface. The angle between the tangent to the curved surface of the free end at any point and the axis of revolution of the transformer determines (in accordance with the thickness of the liquid layer) the extent of transfer of energy to the liquid in the various stages of vibration and the radius of curvature of the curved surface determines the thickness of the liquid layer. More viscous liquids require much more acoustic energy and hence for rapid transfer of energy to the liquid the initial curvature must be: greater.

FIG. 2 shows to a larger scale a modified form of the end surface suitable for liquids of between 10 and 20 cp. of viscosity. The smaller initial radius of curvature of the surface can lead to the existence of regions which can trap droplets and disrupt flow. In order to avoid this, the shape of the surface towards the axis is modified, so that a pip 20 is formed at the extremity. Near the pip there is thus obtained a point of inflection in the axial sectional view shown. More particularly, the ogive is defined by a conoidal concave surface 20 which is faired into the surface of a convex frusto-conical portion 10 of the end surface; wherein the surface 10 is contiguous with and in faired relation to the cylindrical body section 3. The conoidal concave surface 20 provides the inflection point or pip.

When the viscosity of the liquid is even higher than 10-20 cp., for example in the range of 20 to 60 cp. it is possible with a limb 3 as shown in FIG. 2 for a stream of droplets to occur at the centre of the atomised spray, this stream coming from the pip at the extreme end of the transformer limb.

To provide a better atomisation pattern for liquids having a viscosity of 20-60 cp. it is found that a surface having substantially the profile shown in FIG. 3 is satisfactory. The flared end 30 of the limb 3 results in more complete atomisation, any remaining droplets being blended into the rest of the atomised spray. The ogive 3 of FIG. 3 is defined by the surface of a convex frustoconical portion 10, which is contiguous with the cylindrical body 3, and a substantially cylindrical portion 30 having a concave surface. The surfaces of convex portion and concave portion 30 are in faired relationship.

In practice the limb 3 of the device of FIG. 1 has a diameter of 3.5 mms. and the curved portion 10 has a length in the axial direction of 2.5 mms. With FIG. 2 these dimensions were 8.8 and 6.5 mms. respectively and with FIG. 3 12.5 and 8.8 rnms. respectively.

The invention is not limited to the exact arrangement above described, since changes can be made to suit particular circumstances as they arise in practice, for example instead of a plurality of separate feed pipes, the liquid to be atomised may be fed to the vibrating end surface of the velocity transformer through an annular opening at the end of a pair of substantially coaxial feed pipes. Moreover, the liquid feed pipes or the annular opening may be provided with (a) mouth piece(s) so that one or a plurality of liquid jets are obtained.

What is claimed is:

1. An apparatus for ultrasonic atomization of liquids comprising, a velocity transform-er having a cylindrical body portion terminating in a free end face, said end face having an ogival shape fairing into said body portion and means adjacent said cylindrical body for applying a thin film of liquid to the surface of said free end face whereby vibration of said free end face will atomize said liquid film.

2. In an ultrasonic atomiser according to claim 1, wherein said means for feeding a thin film comprises a 4,. plurality of nozzles radially spaced about said body portion and directed toward said ogival end face remote from the apex of said ogive.

3. An apparatus according to claim 1, wherein the radius of curvature defining the ogival end face on said velocity transformer is greater than the radius of said cylindrical body portion.

4. An apparatus according to claim 1 wherein said ogival end face is defined by a conoidal concave surface which is in faired relation to a convex frusto-conical surface, said convex surface is in faired relation to said cylindrical body portion and the apex of said concave surface provides an inflection point.

5. An apparatus according to claim 1 wherein said end face is defined by a convex frusto-conical surface which is contiguous with and in faired relation to said cylindrical body portion, said convex surface converging into a substantially cylindrical portion having a concave surface, with said surfaces being in faired relation.

References Cited UNITED STATES PATENTS 2,908,443 10/1959 Fruengel 239102 2,949,900 8/1960 Bodine 239102 3,019,661 2/1962 Welkowitz 181.5 3,114,654 12/1963 Nishiyama et al. 239102 3,145,931 8/1964 Cleall 239-402 3,155,141 11/1964 Doyle et al. 2394 EVERETT W. KIRBY, Primary Examiner. 

1. AN APPARATUS FOR ULTRASONIC ATOMIZATION OF LIQUIDS COMPRISING, A VELOCITY TRANSFORMER HAVING A CYLINDRICAL BODY PORTION TERMINATING IN A FREE END FACE, SAID END FACE HAVING AN OGIVAL SHAPE FAIRING INTO SAID BODY PORTION AND MEANS ADJACENT SAID CYLINDRICAL BODY FOR APPLYING A THIN FILM OF LIQUID TO THE SURFACE OF SAID FREE END FACE WHEREBY VIBRATION OF SAID FREE END FACE WILL ATOMIZE SAID LIQUID FILM. 